net: phy: at803x: Allow specifying the RGMII RX clock delay via phy mode

at803x currently automatically enables the RGMII TX clock delay when the
phy interface mode is PHY_INTERFACE_MODE_RGMII_TXID. The same should be
done when PHY_INTERFACE_MODE_RGMII_ID is specified.
Use a similar logic to enable the RGMII RX clock delay as well.
at803x_context_{save,restore} were not touched because these are only
used on AR8030 which is a RMII phy (RGMII clock delays are irrelevant).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>